Intracellular pathogens, such as Salmonella typhi, the cause of typhoid fever, are particularly adept at resisting killing by our own phagocytic cells. The long term objective of this proposal is to understand Salmonella virulence particularly how this bacteria can survive within macrophages. My laboratory has identified many mutations in genes that are required for intracellular survival and replication. These mutations are located in about 20 genes. We wish to continue our detailed analysis of these genes and their role in pathogenesis. The mutations will continue to be mapped, the genes cloned and sequenced, and the role of the gene product in virulence identified. We have also identified a new virulence factor, an hemolysin, that is necessary for S. typhimurium to cause infection in a mouse model. We wish to continue work on the hemolysin and determine the precise role of the hemolysin in virulence. Last of all, we have determined that at least 10% of the total Salmonella genome is required for virulence and identified mutations in many of these genes. We wish to complete this study by a more detailed analysis of the most interesting of these. S. typhimurium is an ideal organism to study bacterial pathogenesis. It is well defined genetically, it has a simple and inexpensive animal model, the mouse, and it can be easily grown without loss of virulence. This work should lead to a better understanding of other intracellular pathogens and may also result in improved animal and human vaccines.